Thermal bonding structure and manufacture process of flexible printed circuit board

ABSTRACT

A thermal bonding structure and manufacture process of a flexible printed circuit (FPC) board are disclosed, and the thermal bonding structure includes a laminated structure having a first insulating layer with a solder pad area and showing parts of a first conductive layer, the first conductive layer, a second insulating layer, a second conductive layer, and a third insulating layer with a bonding area such that a part of the second conductive layer is exposed, and at least a through hole passing through the first conductive layer to the second conductive layer for propagating heat energy to fuse a solder. Accordingly, the reduction of heat energy lost in the third insulating layer improves the bonding quality, shortens the bonding period, and maintains the material stability under high temperature resulted from high heat energy.

RELATED APPLICATIONS

This application is a Divisional patent application of co-pendingapplication Ser. No. 11/085,097, filed on 22 Mar. 2005. The entiredisclosure of the prior application Ser. No. 11/085,097, from which anoath or declaration is supplied, is considered a part of the disclosureof the accompanying Divisional application and is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention generally relates to a flexible printed circuitboard, and more particularly relates to a thermal bonding structure andmanufacture process of a flexible printed circuit board that can improvethe issues of bonding burns and material quality.

BACKGROUND OF THE INVENTION

As our living standard is improving gradually, various electronicconsumer products are introduced to meet extensive consumerrequirements, and thus promoting the prosperity of various industriesdirectly and driving the growth of related sub-industries indirectly. Tofurther meet the consumer requirements and trends for various functions,portability, operability and appearance, in hope of improving consumer'swillingness to buy and brand loyalty, various electronic consumerproducts tend to be designed thinner and lighter. For example, themarket share of color mobile phones with a photographic function andother combined functions grows drastically, and the demand of color LCDpanels and camera modules for mobile phones rises accordingly. Color LCDpanel industry is divided into the area of color super twisted nematic(CSTN) LCDs and thin-film transistor (TFT) LCDs, and the key componentsincluding light emitting diodes (LEDs) and flexible printed circuit(FPC) boards also grow with the high demand for flexibility, 3-D circuitlayout and light weight of a miniaturized foldable design of mobilephones. The estimated quantity of flexible printed circuit boards usedin a color mobile phone is increased from 3-4 pieces to 6-7 pieces, andthe design of flexible printed circuit boards tends to follow a high-endsmall circuit specification. A flexible printed circuit board is made byraw materials including a flexible insulating substrate material and acircuit conductor material (generally copper clad), and the rawmaterials are divided into resins, copper clads, adhesives, coverlays,and flexible copper cladded laminates (FCCL). Since polyimide (PI) hasgood expansibility and heat resistance, therefore PI is generally usedas a resin material and serves as a middle layer and a substrate in themanufacture of flexible copper substrates and also as a coverlay film.

PI manufacturers can produce different PI films from different PImonomers according to different technologies in three main aspects:formula, manufacture process and processing method, and thus differentmanufacturers achieve different applications and performance of thematerials. Further, the flexible copper substrate is divided into twomain types: an adhesive three-layer structure and an adhesivelesstwo-layer structure, and both adopt different manufacture processes,methods and applications, and thus the properties of the materials aredifferent. In general, the adhesive three-layer structure is usuallyapplied to the production of a large number of flexible printed circuitboard products and the adhesiveless two-layer structure is usuallyapplied to the manufacture of high-end flexible printed circuit boards,such as the rigid and flexible printed circuit boards and some of themulti-layer boards. It is believed that the adhesiveless two-layerstructure will take over some of the adhesive three-layer flexiblecopper cladded laminates used for the flexible printed circuit boardswith high resolution and good dimensional stability.

Referring to FIG. 1, a schematic view of the relation between the rawmaterials and the finished goods of a prior art flexible printed circuitboard is illustrated. In the manufacture of the flexible printed circuitboard 150, an insulating substrate material 100 and a circuit conductormaterial 110 are used to produce an adhesiveless two-layer flexiblecopper cladded laminate 130 first, and then a coverlay, a stiffener, andan anti-static layer are used to produce the flexible circuit board 150.On the other hand, an adhesive three-layer flexible copper claddedlaminate 140 is made of an insulating substrate material 100, a circuitconductor material 110 and an adhesive 120, and a flexible printedcircuit 150 is made of such laminate 140. At present, flexible printedcircuit boards are generally used in electronic products, particularlymobile phones and LCDs showing a drastic a growth of using flexiblecircuit boards in their applications.

Referring to FIG. 2, a top view of a flexible printed circuit board anda cross-sectional view of a bonding head according to a prior art areillustrated. The flexible printed circuit board 2 comprises a firstinsulating layer 200, an adhesive layer 210, a conductive layer 220 anda second insulating layer 240; wherein the first insulating layer 200and the second insulating layer 240 are made of the same material ordifferent materials, and the first insulating layer 200 includes asolder pad area 270 and the second insulating layer 240 includes abonding area 250, such that a bonding head 260 is in direct contact withthe bonding area 250 for soldering the flexible printed circuit board 2with another flexible printed circuit board. In actual practices, theboding area 250 is usually situated at a position substantially parallelto the solder pad area 270, so that heat energy can be conducted fromthe bonding area 250 to the adhesive layer 210, conductive layer 220 andsolder pad area 270 for bonding. However, it is necessary to increasethe temperature of the bonding head 260 for bonding, and the hightemperature will burn the first insulating layer 200 and the adjacentadhesive layer 210 black, and thus causing poor bonding quality andappearance of the product, or even deteriorating the materials in thebonding area. For example, a bonding machine sets a temperature for thebonding head for a thermal compression, and the temperature of thebonding head is set to 330° C. for a predetermined time (such as 3seconds for temperature rise) and then the operating temperature of thebonding machine is set to 470° C. for another predetermined time (suchas 3.5 seconds for the bonding), then the solder will be fused tocomplete the bonding process. However, the first insulating layer 200and its adjacent adhesive layer 210 will be burned black at thetemperature of 470° C., and such phenomenon is particularly severe forlead-free solders because the melting point of lead-free solders ishigher than that of lead solders. For example, the melting points of thesolders of the Sn—Ag—Cu series and Sn—Cu—Ni series are 227° C. and 217°C. respectively. Compared with the melting point 183° C. of solder ofthe Sn—Pb series, there is a difference of 34˜44° C. Therefore, it isnecessary to increase the temperature of the bonding head 260 forlead-free solders in compliance with the environmental protection andinternational standard requirements. As a result, the burning effectproduced in the bonding area 250 becomes obvious and severe.

Therefore, developing a thermal bonding structure and manufactureprocess for a flexible printed circuit board to overcome the foregoingshortcomings of the prior arts, improving the burning situation in thebonding area, and further conducting heat energy to the solder so as tolower the bonding temperature and supply less heat energy for savingbonding time and costs are important topics for manufactures and usersand demand immediate attentions and feasible solutions. The inventor ofthe present invention based on years of experience on related researchand development of the optoelectronic component industry to invent athermal bonding structure and manufacture process for flexible printedcircuit boards to overcome the foregoing shortcomings.

SUMMARY OF THE INVENTION

Therefore, it is a primary objective of the present invention to providea thermal bonding structure of a flexible printed circuit board thatcomprises: a laminated structure and the laminated structure includes afirst insulating layer, a first conductive layer, a second insulatinglayer, a second conductive layer and a third insulating layer insequence; at least a through hole for passing through the firstconductive layer, the second insulating layer and the second conductivelayer. The first insulating layer includes a solder pad area forexposing the first conductive layer, and the third includes a bondingarea for exposing the second conductive layer, such that the bondinghead is in direct contact with the bonding area, and the heat energy canbe conducted to the solder pad area through the through hole quickly forbonding.

Another objective of the present invention is to provide a flexibleprinted circuit board comprising a first area, and the first areaincludes a laminated structure having a first insulating layer, a firstconductive layer, a second insulating layer, a second conductive layerand a third insulating layer arranged in sequence; a second areaincluding a laminated structure coupled to the first area and having afirst insulating layer, a first conductive layer, and a secondinsulating layer arranged in sequence; and a third area including alaminated structure disposed away from the first area and coupled to thesecond area and having the foregoing first insulating layer, firstconductive layer, second insulating layer, second conductive layer andthird insulating layer arranged in sequence; and at least one throughhole passing through the foregoing first conductive layer, secondinsulating layer and second conductive layer. The first insulating layerin the third area includes a solder pad area for exposing the firstconductive layer and being in contact with the solder, and the thirdinsulating layer includes a bonding area for exposing part of the secondconductive layer to define a bonding area, such that the heat energy ofthe bonding head can be conducted to the solder pad area through thethrough hole quickly to reduce bonding time and heat supply costs.

A further objective of the present invention is to provide a manufactureprocess of a flexible printed circuit board that comprises the steps of:providing a laminated structure and the laminated structure is dividedinto a first area, a second area and a third area, and the second areais disposed between the first area and the third area, and the laminatedstructure in the first and third areas includes a first insulatinglayer, a first conductive layer, a second insulating layer, a secondconductive layer and a third insulating layer arranged in sequence, andthe second area includes a first insulating layer, a first conductivelayer, a second insulating layer, and at least one through hole formedin the first area and third area separately and passing through thefirst conductive layer, the second insulating layer and the secondconductive layer; removing a part of the third insulating layer toexpose the second conductive layer and define a bonding area; removing apart of the first insulating layer in the third area to expose the firstconductive layer and define a solder pad area; and removing a part ofthe third insulating layer in the third area to expose the secondconductive layer and define a bonding area.

Thus, the thermal bonding structure and manufacture process of aflexible printed circuit board in accordance with the present inventionhas the following advantages. Since the heat consumption at theinsulating layer is reduced, therefore the bonding head can achieve thebonding effect with less heat energy and the cost for the bondingprocess can be lowered. Furthermore, the bonding head is applied to thebonding area, and the through hole is used to conduct heat energy to thesolder pad area to accomplish the bonding process, and thus thetemperature of the bonding head can be controlled to improve the burningphenomenon caused by the high temperature of the bonding head andoccurred at the bonding area, so as to enhance the soldering process,the material quality, and the appearance of the product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the relation between the raw materials andthe finished goods of a prior art flexible printed circuit board;

FIG. 2 shows a top view of a finished goods of a flexible printedcircuit board and a cross-sectional view of a bonding head according toa prior art;

FIG. 3 shows a top view of a thermal bonding structure of a flexibleprinted circuit board and its corresponding cross-sectional viewaccording to a preferred embodiment of the present invention;

FIG. 4 shows a top view of a flexible printed circuit board and itscorresponding cross-sectional view according to a preferred embodimentof the present invention; and

FIG. 5 shows a flow chart of the manufacture process of a flexibleprinted circuit according to a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To make it easier for our examiner to understand the objective of theinvention, its innovative features and performance, a detaileddescription and technical characteristics of the present invention aredescribed together with the drawings as follows.

Referring to FIG. 3, a top view of a thermal bonding structure of aflexible printed circuit board and its corresponding cross-sectionalview according to a preferred embodiment of the present invention areillustrated. In the preferred embodiment, a thermal bonding structure 3of a flexible circuit board comprises: a laminated structure having afirst insulating layer 310, an adhesive layer 320, a first conductivelayer 330, an adhesive layer 320, a second insulating layer 340, anadhesive layer 320, a second conductive layer 350, and a thirdinsulating layer 360; and at least one through hole 380 passing througheach layer between the first conductive layer 330 and the secondconductive layer 350. The first insulating layer 310 includes a solderpad area 390 for exposing the first conductive layer 330, and the thirdinsulating layer 360 includes a bonding area 365 for exposing the secondconductive layer, and the through hole 380 is formed beyond the range ofthe solder pad area 390 and the bonding area 365. In other words, thereis a gap between the solder pad area 390 and the bonding area 365.Referring to FIG. 3 for a cross-sectional view of a thermal bondingstructure of a flexible printed circuit board according to a preferredembodiment of the present invention, the through hole 380 is formedbeyond the range of the solder pad area 390 and the bonding area 365. Itis noteworthy that each conductive layer can be stacked on top of eachinsulating layer directly as disclosed in another embodiment, since thelaminated structure of the thermal bonding structure of the flexibleprinted circuit board can only have a first insulating layer 310, afirst conductive layer 330, a second insulating layer 340, a secondconductive layer 350 and a third insulating layer 360. The through hole380 of this embodiment includes an electric conductive material on itsinternal wall, and the materials used for making the first conductivelayer 330 and the second conductive layer include a copper clad, and thesurface of the second conductive layer 350 of the bonding area 365further includes a metal layer for protecting the second conductivelayer 350. The metal layer could be single-layer or multiple-layer andmade of gold, nickel, tin, other metal, or an alloy of the foregoingmetals. In this embodiment, a nickel layer and a gold layer are formedin sequence on the surface of the second conductive layer 350 of thebonding area 365, and the materials used here are provided for thepurpose of describing the present invention and not intended to limitthe invention. Further, the quantity and size of the through holes 380vary with the speed and time of the heat conduction, and thus thepositions of the through holes and the bonding area described in thisembodiment are provided for example only, and not limited to the samenumber and size of the through holes 380 as depicted in FIG. 3.

A lithographic etching is adopted to remove a part of the thirdinsulating layer 360 to define a bonding area 365, so that heat energycan be conducted directly and quickly from the second conductive layer350 to the first conductive layer 330 and fuse a solder such as a solderpaste in the solder pad area 390. With the same conditions as the priorart, a solder can be fused to complete a bonding process with a lowertemperature, if the temperature of the bonding machine is set to 330° C.for a predetermined time (such as 3 seconds for temperature rise) andthe operating temperature of the bonding machine to 400° C., and thebonding head 370 is operated for a bonding time (such as 3.5 seconds),and thus improving or avoiding the burning phenomenon at the bondingarea 365 and the third insulating layer 360.

Referring to FIG. 4, a top view of a flexible printed circuit board andits corresponding cross-sectional view according to a preferredembodiment of the present invention are illustrated. In this embodiment,the flexible printed circuit board 4 is divided into a first area 410, asecond area 430 coupled to the foregoing first area 410 and a third area450 disposed away from the first area 410 and coupled to the second area430. The flexible printed circuit board 4 comprises a laminatedstructure, and the first area 410 (which is a connecting area forcomponents such as the light emitting diodes) and the third area 420include a first insulating layer 411, a first conductive layer 412, asecond insulating layer 413, a second conductive layer 414 and a thirdinsulating layer 415 arranged in sequence, and the second area 430includes a first insulating layer 431, a first conductive layer 432, asecond insulating layer 433 arranged in sequence and considered as acircuit area; and at least one through hole 458 passing through thefirst conductive layer 452, the second insulating layer 453 and thesecond conductive layer 454. The first insulating layer 451 in the thirdarea 450 includes a solder pad area 456 for exposing the firstconductive layer 452, and the third insulating layer 455 includes abonding area 457 for exposing the second conductive layer 454 andcontacting a bonding head 459. The surface of the second conductivelayer 454 exposed from the bonding area 457 could include a metal layer,which is a single-layer metal layer such as a gold layer, or adouble-layer metal layer such as a nickel layer and a gold layer. Thesematerials are used for example to describe the present invention and notintended to limit the invention.

It is noteworthy that the through hole 458 is formed beyond the range ofthe solder pad area 456 and the bonding area 457, and the through hole458 includes an electric conductive material such as nickel or any othersubstance having a thermal conductive property. In addition, thematerials used for the first conductive layer 452 and the secondconductive layer 454 could include a copper clad, and an adhesive layercould be included between layers.

Referring to FIGS. 4 and 5, a flexible printed circuit board and a flowchart of a manufacture process of a flexible printed circuit boardaccording to a preferred embodiment of the present invention areillustrated. In the embodiment, the manufacture process of a flexibleprinted circuit board 4 comprises the steps of: (Step S51) providing alaminated structure, which is a four-layer structure as shown in thefigure, and the laminated structure is divided into a first area 410, asecond area 430 and a third area 450, and the second area 420 isdisposed between the first area 410 and the third area 450 and includesa first insulating layer 411, a first conductive layer 412, a secondinsulating layer 413, a second conductive layer 414 and a thirdinsulating layer 415; and at least one through hole 458 being formed atthe first area 410 and the third area 450 and passing through the firstconductive layer 412, second insulating layer 413 and second conductivelayer 414; (Step S52) removing a part of the first insulating layer 411in the third area 450 to expose the first conductive layer 412 to definea solder pad area 456; and (Step S53) removing a part of the thirdinsulating layer 415 in the third area 450 to expose the secondconductive layer 414 to define a bonding area 457.

In another preferred embodiment of the present invention, a manufactureprocess of a flexible printed circuit board further comprises the stepsof removing the second conductive layer 414 and the third insulatinglayer 415 in the second area 430, such that the laminated structure inthe first area 410 and the third area 450 includes a first insulatinglayer 411, a first conductive layer 412, a second insulating layer 413,a second conductive layer 414 and a third insulating layer 415 arrangedin sequence, and the laminated structure in the second area 430 includesa first insulating layer 411, a first conductive layer 412 and a secondinsulating layer 413 arranged in sequence; depositing an electricconductive material in at least one through hole 458 after forming atleast one through hole 458 that passes through the first conductivelayer 412, the second insulating layer 413 and the second conductivelayer 414; and forming a metal layer on the surface of the bonding area457 after forming the bonding area 457. It is noteworthy that thelaminated structure further comprises at least one adhesive layerdisposed between the first insulating layer, the first conductive layer412, the second insulating layer 413, the second conductive layer 414and the third insulating layer 415.

While the invention has been described by way of example and in terms ofa preferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

In summation of the description above, the present invention is noveland useful and definite enhances the performance over the conventionalstructure and further complies with the patent application requirementsand is submitted to the Patent and Trademark Office for review andgranting of the commensurate patent rights.

1. A method for manufacturing a flexible printed circuit board,comprising the steps of: providing a laminated structure having a firstinsulating layer, a first conductive layer, a second insulating layer, asecond conductive layer and a third insulating layer laminated insequence; dividing the laminated structure into a first portion, asecond portion and a third portion, wherein the second portion isbetween the first portion and the third portion; forming at least onethrough hole in the second insulating layer of the first portion and thethird portion, respectively; forming a solder pad area by removing apart of the first insulating layer of the third portion to expose thefirst conductive layer; and forming a bonding area by removing a part ofthe third insulating layer of the third portion to expose the secondconductive layer, wherein the at least one through hole is formed beyondthe range of the solder pad area and the bonding area.
 2. The method ofclaim 1, further comprising a step of: removing the second conductivelayer and the third insulating layer of the second portion.
 3. Themethod of claim 1, wherein the step of forming at least one through holefurther comprises a step of: forming a conductive material in the atleast one through hole.
 4. The method of claim 1, wherein the firstconductive layer and the second conductive layer are comprised ofcopper.
 5. The method of claim 1, further comprising a step of: formingan adhesive layer disposed between the first insulating layer and thefirst conductive layer.
 6. The method of claim 1, further comprising astep of: forming an adhesive layer disposed between the first conductivelayer and the second insulating layer.
 7. The method of claim 1, furthercomprising a step of: forming an adhesive layer disposed between thesecond insulating layer and the second conductive layer.
 8. The methodof claim 1, further comprising a step of: forming an adhesive layerdisposed between the second conductive layer and the third insulatinglayer.